The present invention relates to a multi-input multi-output (MIMO) receiver and a method of reception as may be used in a concurrent transmission on a common frequency band and which apply an adaptive equalization processing to received signals received from N transmitters (where N is an integer equal to and greater than 2) by way of M antennas (where M is an integer equal to or greater than 2) for purpose of decoding processing.
A task in the mobile station communication business is how to construct a system capable of accommodating for a multitude of users on a limited frequency band with a high quality. A multi-input multi-output (MIMO) system is known in the art as means for solving such a task. In this system, a first to an N-th transmitter use a common frequency band to transmit a first to an N-th symbol sequence signal, respectively, during a common time interval. These transmitted signals are received by an MIMO receiver equipped with a plurality of antennas #1 to #M. The received signals are processed by the MIMO receiver, which estimates the first to the N-th transmitted signal from the first to the N-th transmitter and delivers them to output terminals Out-1 to Out-N separately for each transmitter.
An arrangement of a single transmitter in the MIMO system is shown in FIG. 1. An error correction coding is applied to information bits in a channel coder 51, and a resulting coded bit sequence is interleaved in an interleaver means (INTERLE.) 52 before it is mapped to symbol in a symbol mapping means 53 for transmission. The transmission takes place after adding a training symbol sequence which is already known to the receiver side and which is specific to each transmitter and fed from a training symbol generator means 54 to the symbol sequence from the mapping means 53 at a position which precedes the symbol sequence in a multiplexer means 55. The signal sequence to be transmitted is converted into a high frequency signal to be radiated as a radio wave from an antenna.
MIMO receiver is disclosed, for example, in European laid open patent application EP12335655A2 (issued Aug. 21, 2002). The arrangement of a conventional MIMO receiver is shown in FIG. 2. Received signals from antennas #1 to #M are converted to baseband signals, fed as received signal r1 to rM to be subject to an equalization processing in an MIMO adaptive equalizer 35, and an output from the MIMO adaptive equalizer 35 which corresponds to each transmitted sequence is decoded in one of decoders 621 to 62N, respectively. The arrangement of the MIMO adaptive equalizer is shown in FIG. 3. Received signals r1 to rM which are sampled from the baseband signal, a priori information sequence 2 (λ21 to λ2N) which is fed back from an SISO decoder 63 and a channel status (propagation path response) estimate H are input to the adaptive equalizer 35. A replica of the undesired received signal is reproduced in a replica generator (REP. GEN.) 65 using the a priori information sequence 2 and the channel estimate. A subtractor 66 subtracts the replica from the received signal, thus reducing ISI (inter-symbol interferences) an MAI (inter-channel interferences). ISI and MAI which remain after this reduction processing are processed by an MMSE filter 67, and the logarithmic likelihood ratio of each symbol is calculated by an LLR generator 68 from the result of such processing to deliver a priori information sequence 1 (λ11 to λ1N). In the decoders 621 to 62N in FIG. 2 which corresponds to each user (transmitter), the a priori information sequence 1 of each symbol which is calculated by the MIMO adaptive equalizer 35 is transformed into a bit sequence by a demapping part not shown in the drawing, and the bit sequence is deinterleaved in a deinterleaver (DEINTER.) 69, and the deinterleaved sequence is decoded by the SISO decoder 63. A soft decision output value sequence from the SISO decoder 63 is interleaved by an interleaver (INTERLE.) 71 and the output of the interleaver 71 is mapped by a mapping part not shown in the drawing and to obtain a soft decision symbol sequence which is supplied as a priori information sequence 2 (λ21 to λ2N) to the MIMO adaptive equalizer 35. The described processings in the MIMO adaptive equalizer 35 and the SISO decoders 621 to 62N are repeated a plurality of times, and an eventual decoding result is delivered as a decided bit sequence Bseq1 to BseqN. In the case where a symbol sequence is interleaved in the transmitting side, the priori information sequences 1 and 2 of the symbol sequence are also subjected to deinterleave and interleave, respectively, in the SISO decoders.
In order to achieve a satisfactory equalization in the MIMO adaptive equalizer 35 or to ensure that the inter-symbol interferences and inter-channel interferences be satisfactorily eliminated, it is important to detect a sampling timing of a received signal, a sync timing and to determine the duration of the equalization, or in other words, to detect a sync timing inclusive of a frame sync and symbol sync. However, this point is not disclosed in the European laid open patent application cited above. A conventional adaptive equalizer which would be used when a single user (transmitter) exclusively uses one channel (transmission path) for transmission is shown in FIG. 4. In order to form an equalization start timing signal, an equalization duration signal and a channel status estimate, a sync channel generator 81 precedes the adaptive equalizer 61. While not shown, a transmitting side initially transmits a long training symbol sequence (sync word signal), the transmitted symbol pattern of which is already known to a receiving side during each frame, and then transmits data representing an information content which is to be transmitted. While not shown, on the receiving side, a radio wave received from the transmitting side is received by an antenna, amplified and demodulated to be converted into a baseband received signal, which is then input to an input terminal 11. A sampling signal generator 19 forms a correlation between the received signal and a training symbol sequence TSS which is fed from a training sequence generator (TSS GEN.) 13, and a sampling timing is chosen at the time when the correlation value assumes a maximum value or when a received signal reaches from a path which provides a high received signal power. A received signal which is obtained with a sampling signal that is generated by the sampling signal generator 19 at this sampling timing is provided from a sampler 20 as a received signal having a digital sequence which is sampled. The correlation between the received signal having a digital sequence and the training symbol sequence TSS fed from the training sequence generator 13 is formed by a sync timing generator (TIMING GEN.) 12. The correlation signal produced by the sync timing generator 12 changes in a manner as illustrated in FIG. 5A, for example, and a timing ts is detected where the correlation signal Sigc is at its maximum, and is used as the sync timing ts. The sync timing signal which is produced at the timing ts, the digitized received signal, and the training symbol sequence TSS are input to a start timing/duration candidate signal generator 21 (START/DURA GEN.), which provides an equalization start timing signal and an equalization duration signal, both of which are input together with a sampled received signal to a channel estimator 28 (CHAN.EST.GEN.), which then estimates a transmission path response (channel status). The estimated channel status, the sampled received signal, the equalization start timing signal and the equalization duration signal are then input to an adaptive equalizer 61 where the sampled received signal is subject to an adaptive equalization processing to deliver a decided symbol sequence.
As shown in FIG. 5B which illustrates the equalization start timing ts and the equalization duration TE, of paths (or received signal samples) located across Ns symbols centered about the detected sync timing ts, those paths (samples) having a correlation output Sigc with the training symbol sequence which exceeds a given threshold value TH1 are designated as effective paths, the leading effective path (sample) position is defined as the equalization start timing t0 and the equalization duration signal TE is delivered as extending from the leading effective path (sample) to the last effective path (sample) position te. To determine which one of the paths represents an effective path, the threshold value TH1 may be chosen to be 1/Cp times a maximum correlation power MAX among the paths (samples) where Cp represents a predetermined constant, and samples equal to or exceeding TH1 may be determined as effective paths.
Up to the present time, a sufficient investigation is not made into the manner of determining the sync timing, the equalization start timing and the equalization duration in the MIMO receiver.
It is an object of the present invention to provide an MIMO receiver and an MIMO reception method which enable efficient detection of at least equalization start timing t0 allow a satisfactory adaptive equalization processing even when there are many signal sequences (users).